This invention relates to manually operated or actuator positioned modulating or shut-off valves to control the flow of fluid in a pipe conduit. The invention is especially suitable for use with sanitary media or highly corrosive fluids. Prior art of reference is my U.S. Pat. No. 4,214,730 which teaches the art of constructing a reciprocating plug valve cooperating with Teflon (tetrafluoroethylene) that is retained within a stainless steel housing. One major disadvantage of this prior construction is that it requires a stem packing seal which is subject to normal wear and could cause environmental problems due to stem leakage of acids or other dangerous chemicals handled by the valve.
Another prior art solution is shown in my U.S. Pat. No. 4,609,178. In this invention the stem leakage is eliminated by the use of an elastomeric diaphragm which cooperates with a metal valve seat. The disadvantage here is that, in order to be corrosion resistant, the valve housing has to be made from an expensive metal alloy. Also, due to problems with geometry, the design is not suitable for very small flow rates such as found in small pH control applications.
The present invention overcomes these and other objections as follows:
First, by using a cylindrical insert, within a pressure supporting metal housing, which is surrounded by a tube that simultaneously acts as stem seal and as valve closure member. Both the cylindrical insert and the sealing tube can be made from highly corrosion resistant plastic or from food grade type elastomers.
Secondly, the flow capacity of this valve can readily be changed by the interchange of a cylindrical insert with a different orifice or port size.
Thirdly, since the sealing tube is only slightly displaced around its central axis, there is very little stress (in contrast to stem sealing bellows) guaranteeing a long life cycle.
Fourthly, the inherent simplicity of design insures a low manufacturing cost and parts that are free of critical tolerances.
Finally, fine flow control is assured without the need of metal parts being in contact with the fluid.